The present invention relates to a method for producing wear-resistant layers on materials of barrier-layer-forming metals, such as in particular aluminum, magnesium and titanium and their alloys and mixtures, by means of laser treatment and to the application of this method and to the materials provided with wear-resistant layers produced in this way.
The production of wear-resistant layers on materials of barrier-layer-forming metals, such as aluminum, magnesium and titanium and their alloys, under electrolytic conditions is known: for instance, wear-resistant layers with excellent properties can be obtained by what is known as anodic oxidation with spark discharge (known as the ANOF method) and suitable, usually aqueous or aqueous-organic electrolyte solutions. Such a method is described for example in EP 0 545 230 B1. A disadvantage of these methods is that they work electrolytically and therefore use electrolyte baths, which subsequently require disposal. What is more, after their production, the layers produced must be cleaned of undesired constituents of the electrolyte bath. Therefore, efforts are increasingly being made to produce such wear-resistant layers in some other way.
The multitude of applications for surface refinement in automobile construction and other areas, in particular in the area of mechanical engineering, is sufficient testimony to the need for technologies that meet the increased requirements for the functionality of the components. Laser methods offer new approaches here to improving the quality of the components. However, wear-resistant layers have a leading role to play. In principle, the use of lasers for surface treatment opens up new environmentally friendly technologies, in particular since they do not need electrolyte baths.
In DE 102 02 184 C1 and the journal HTM 52 (1997) 2, pages 91 to 93 (J. Barnikel et al. “Nitrieren von Aluminiumlegierungen mit UV-Laserstrahlung” [nitriding of aluminum alloys with UV laser radiation], comments are made on the laser nitriding of aluminum surfaces.
For instance, DE 102 02 184 C1 describes a method for producing wear-resistant layers in regions of components that are near the surface, in particular pistons for internal combustion engines, from a composite aluminum base material, at least parts of the surface of the components having undergone hardening and the wear-resistant layer being formed from aluminum nitrides in an aluminum matrix, the wear-resistant layer being produced by means of a laser nitriding treatment, with energy being introduced into the surface in the form of pulses, so that a remelt layer forms in the areas near the surface and this causes a conversion of nitrogen from the nitrogen atmosphere or from the air with aluminum from the composite material in such a way that the aluminum nitrides are in a finely dispersed and graded form in the remelt layer.
Although the aluminum nitride (AlN) formed in this way is very hard (about 1230 HV=Vickers hardness), it is also very brittle. It therefore tends to crack and is consequently unusable for many applications, in particular in automobile construction. Particularly safety components that are exposed to vibrations, such as for example aluminum components for internal combustion engines, such as in particular pistons, cylinder faces, valves and the like, are at great risk if they are provided with such an aluminum nitride layer. The use of such components provided with aluminum nitride layers can cause the entire engine to fail while it is running. The layer thickness of the aluminum nitride layer produced is also relatively small. Moreover, under point loading of the surface there occurs an effect known as the “eggshell effect”: the base material under-goes plastic deformation and this causes subsequent crack formation.
The technical teaching of DE 102 02 184 C1 also does not overcome the aforementioned disadvantages, even if the energy of the laser is applied to the aluminum surface in a pulsed manner in a nitrogen atmosphere and the aluminum nitride is formed in a finely dispersed manner.
A further possibility for surface refinement by means of laser treatment is that of producing aluminum materials by the laser treatment of protective oxide-ceramic layers, the material particles, such as for example aluminum oxide (Al2O3), zirconium oxide (ZrO2) etc., being melted onto the surface of the aluminum material (cf. Laser und Optoelektronik, 29(4), pages 48 to 52, 1997). The disadvantage of this possibility in principle of melting solid materials by laser and applying them to the material surfaces concerned is that these particles cannot be applied uniformly to the surface of the material. In particular in the case of components of a complicated shape, uniform coating cannot be accomplished. Furthermore, poor bonding of the melted particles with respect to the material surface is often observed, which is often caused by an already existing oxide layer on the workpiece to be treated.
The problem on which the present invention is based is therefore that of providing a method for producing wear-resistant layers on materials of barrier-layer-forming metals, in particular aluminum, magnesium and titanium and their alloys and mixtures, which largely avoids, or at least mitigates, the disadvantages of the prior art described above.
To solve the problem described above, the present invention proposes—according to a first aspect of the present invention—a method as claimed in claim 1. Further, particularly advantageous refinements of the method according to the invention are the subject of the respective method sub-claims.
This is so since the applicant has now surprisingly found that the problem described above can be solved by exposing the material surfaces of materials based on barrier-layer-forming metals, such as aluminum, magnesium and titanium and their alloys and mixtures, to a laser treatment in the presence of an atmosphere containing oxygen, or laser irradiation in the form of a laser oxidation treatment, in such a way that the upper or outer layer of the material surface is exposed to the oxygen to form an oxide of the metal constituting the material, while the layer of the material lying under that is remelted without reacting with the oxygen.
Further subject matter of the present invention—according to a second aspect of the present invention—is the application according to the invention of the method based on the present invention, as it is defined in the respective claims.
Finally, subject matter of the present invention—according to a further, third aspect of the present invention—is the materials according to the present invention that can be obtained by the method according to the invention, which are provided with a wear-resistant layer of the aforementioned type and as defined in the respective claims.
The subject matter of the present invention is consequently—according to a first aspect of the present invention—a method for producing wear-resistant layers on materials of barrier-layer-forming metals, in particular aluminum, magnesium and titanium and their alloys and mixtures, with preference aluminum or its alloys, by means of laser treatment, the material surface being exposed to a laser irradiation in the presence of an atmosphere containing oxygen in such a way that the upper or outer layer of the material surface is reacted or converted with the oxygen of the atmosphere containing oxygen to form an oxide of the metal constituting the material, while the layer of material lying under that is remelted without reacting with the oxygen.
The laser treatment or laser oxidation according to the invention results in wear-resistant layers with excellent wear-resistant properties, in particular with excellent corrosion resistance and excellent abrasion resistance and extreme hardness, the wear-resistant layers not exhibiting any brittleness—unlike aluminum nitride layers of the prior art—and, because of the hardness gradient within the layer structure—the hardness (Vickers hardness) of the layers or the layer structure decreasing gradually from the outside inward—exhibiting excellent mechanical properties, in particular not having a tendency toward the “eggshell effect” under point loading of the surface.
The layers produced according to the invention have properties that are comparable, or to some extent improved, in comparison with wear-resistant layers produced according to conventional electrolytic methods, with disadvantages being avoided in an efficient way, in particular by avoiding the use of electrolyte baths.
The laser treatment or laser oxidation carried out according to the invention results in a multilayer structure: the actual wear-resistant layer as such generally comprises a two-layer structure, this comprising the upper or outer oxide layer of the metal constituting the material and the layer of remelted material (“remelt layer”) lying adjacent the upper or outer oxide layer and lying under this oxide layer, arranged underneath which there is then the unchanged (i.e. unreacted and not remelted) layer of the material adjacent said remelt layer. Altogether, it therefore results in a multilayer structure which comprises—when viewed from the outside inward or from top to bottom—the upper, outer oxide layer of the metal constituting the material, the remelt layer arranged under that and the layer of unreacted and not remelted base material arranged in turn under that. In this case, the outer layer (i.e. the oxide layer of the metal constituting the material) has the greatest hardness (Vickers hardness), the remelt layer lying under that has a lower hardness (Vickers hardness) in comparison, and the layer of the base material arranged in turn under that has the lowest hardness (Vickers hardness). This produces as desired a multilayer structure with the aforementioned hardness gradient, which leads to excellent mechanical properties.
According to one particular embodiment of the method according to the invention, it may be provided that, before the production of the wear-resistant layer (i.e. before the laser treatment or laser oxidation according to the invention), the material surface is subjected to a remelting, in particular likewise by means of laser treatment, with preference under inert conditions. When doing so, it must be ensured in particular that no oxidation of the material surface takes place during this pretreatment. This is achieved by working under inert conditions, in particular under an inert gas atmosphere, prefera-bly under a noble gas atmosphere, and below the reaction temperatures of the material surface, generally below temperatures of 1.000° C. of the material surface. However, this preceding method step of remelting is of a purely optional nature.
According to a preferred embodiment, aluminum or an aluminum alloy is used in particular as the metal constituting the material, so that an aluminum oxide layer (Al2O3 layer) results as the upper, outer layer of the laser treatment or laser oxidation according to the invention. The material according to the invention may be, for example, a cast or diecast material, in particular an aluminum cast or diecast material. In particular, it may be a coarse-grained cast or diecast material, in particular cast or diecast aluminum material, which may possibly have been subjected to a remelting, in particular likewise by means of laser treatment, as described above, before the production of the wear-resistant layer by the laser treatment according to the invention, this pretreatment being optional. Instead of cast or diecast alloys, wrought alloys, in particular wrought aluminum alloys, may also be subjected to the treatment according to the invention. However, the aforementioned examples of materials used are not of a restrictive nature.
In principle, a laser with a wavelength in the range from 700 to 1.200 nm, in particular 800 to 1.100 nm, is used for the laser treatment according to the invention.
In principle, both pulsed and nonpulsed lasers may be used for the laser treatment or laser oxidation according to the invention. In the case of the use of pulsed lasers, the pulse duration (FWHM) is chosen in particular in the range from 10−7 s to 10−2 s, in particular at approximately 10−3 s; the layer thickness of the wear-resistant layer can be controlled in a specifically selective manner by means of the pulse duration of the laser.
For example, a nonpulsed diode laser or an Nd:YAG laser, in particular respectively at a wavelength in the range from 800 to 1100 nm, may be used within the scope of the method according to the invention.
Generally, the laser treatment is carried out in such a way, in particular the energy that is introduced or made to act by means of laser irradiation is dimensioned in such a way, that the reaction temperature Treaction at the material surface is at least 1.000° C. (Treaction≧1.000° C.).
Generally, the power density used for the laser may vary within broad ranges. For ex-ample, the power density used for the laser may be chosen in the range from 104 to 108 W/cm2, in particular in the range from 105 to 107 W/cm2, preferably at approximately 106 W/cm2. Nevertheless, it may be necessary owing to the individual case or on the basis of the application to deviate from the aforementioned values without departing from the scope of the present invention.
As described above, the laser treatment or laser oxidation according to the invention is carried out in an atmosphere containing oxygen. The atmosphere containing oxygen may either comprise or consist of pure oxygen or comprise or consist of a gas mixture of oxygen with at least one further inert gas that is nonreactive under reaction conditions, preferably a noble gas. In order that no nitrides, in particular no aluminum nitride, can be formed during the laser treatment or laser oxidation according to the invention, the atmosphere containing oxygen does not contain any nitrogen and/or any gas generating nitrogen under reaction conditions.
Generally, the method according to the invention is carried out under atmospheric pressure. Nevertheless, carrying out the method under reduced or increased pressure is not ruled out, even though it is preferred for the method to be carried out under atmospheric pressure.
Wear-resistant layers produced by the method according to the invention generally have total thicknesses of 50 to 350 μm, in particular 75 to 300 μm, preferably 100 to 250 μm. These thicknesses generally comprise the upper or outer oxide layer and the remelt layer lying under it.
As far as the upper or outer layer is concerned, which in the case of aluminum or aluminum alloys is an aluminum oxide layer (Al2O3 layer), possibly with further constituents (for example SiO2 or mullite in the case of silicon-containing aluminum alloys), its layer thickness is generally 1 to 50 μm, in particular 2 to 30 μm, preferably 3 to 20 μm.
The upper, outer layer, in particular aluminum oxide layer (Al2O3 layer) has an extreme hardness. The Vickers hardness (HV) of this upper (outer) layer is at least 1.000 HV, in particular at least 1.500 HV, preferably at least 2.000 HV.
A further, particular feature of this upper, outer layer, in particular aluminum oxide layer (Al2O3 layer), is its extremely low roughness (peak-to-valley height): generally, the roughness (peak-to-valley height) Ra of the upper, outer layer is ≦0.5 μm, in particular ≦0.4 μm, preferably ≦0.3 μm. Consequently, the wear-resistant layers produced according to the invention are also suitable for those applications in which the dimensional stability and evenness of the layers have to meet extremely high requirements.
In the event that the material consists of aluminum or an aluminum alloy, the upper, outer layer of the wear-resistant layer according to the invention is an aluminum oxide layer (Al2O3 layer) and comprises at least 60%, preferably at least 80%, with particular preference at least 90%, corundum (α-Al2O3). This explains the extreme hardness of this outer layer. In the case of silicon-containing aluminum alloys, the outer layer may also contain up to 10%, in particular up to 20%, preferably up to 30%, silicon dioxide (SiO2), preferably in the form of mullite; this likewise exhibits a great Vickers hardness. All the aforementioned figures in percent are given as percent by weight with respect to the weight of the upper, outer layer.
As far as the remelt layer is concerned, arranged under the outer oxide layer, in particular Al2O3 layer, it generally has a thickness in the range from 50 to 300 μm, in particular 75 to 250 μm, preferably 100 to 200 μm.
This remelt layer generally has a Vickers hardness (HV) that is less than the Vickers hardness (HV) of the outer layer lying above it and a Vickers hardness (HV) that is greater than the underlying layer of base material. Generally, the remelt layer arranged under the outer oxide layer, in particular under the outer Al2O3 layer, has a Vickers hardness (HV)≧150 HV, in particular ≧200 HV. The much lower Vickers hardness of the remelt layer in comparison with the outer oxide layer is explained by the fact that the remelt layer was created by the base material simply remelting, but not reacting with the oxygen of the laser treatment atmosphere; a greater Vickers hardness of the remelt layer in comparison with the underlying layer of the base material is in turn ex-plained by the fact that a finely dispersed or finely grained phase or layer is created by the remelting process.
This is so since the remelting process by means of laser treatment or laser oxidation according to the invention has the effect that the remelt layer arranged under the outer oxide layer, in particular under the outer Al2O3 layer, is formed in a finely dispersed and/or finely grained manner, in particular with a grain size <1 μm, preferably <0.5 μm.
By contrast, the base material lying under the remelt layer is generally formed in a coarsely grained and/or coarsely dispersed manner, in particular with a grain size >10 μm, preferably >20 μm.
As described above, the base material arranged under the remelt layer generally has a lower Vickers hardness than the remelt layer lying above it: generally, the Vickers hardness (HV) of the base material layer lying under the remelt layer is up to 150 HV, and lies in particular in the range from 50 to 150 HV, preferably 75 to 125 HV.
The basic layer structure of the wear-resistant layers obtainable by the method according to the invention is illustrated in the representations of the figures.